The present invention relates generally to data transmission systems, and more particularly to systems which encode digital data onto the scan lines of a television video signal.
It is commonplace today for information to be transmitted over great distances by cable and by radio including satellite relay. Television programs originating in other countries are routinely made available to viewers in this country by way of local ground stations which receive the programs by a microwave relay link including a satellite, and distribute the received signals over a cable in a form which can be reproduced over the viewer's television receiver.
The present availability of means for distributing television signals nearly instantaneously all over the world has given rise to a number of systems for disseminating data in digital form along with the television signals, e.g., during the vertical blanking interval between each scanned video field, so that information other than the video signal can be received by private users. Such information may include news wire service, weather maps or any other information which has previously been distributed only by wire or relatively short radio paths. Examples of these systems include the French DIDON Transmission System, The British Teletext System, and the Canadian Telidon Terminal.
All of the above systems are known as "transparent" in that they provide a faithful replica of digital data originating from an input device (e.g., teletype, computer output) at one end of the communications path to a suitable output device (e.g., teletype, video terminal) at the other end of the path. Further, these systems use a standard television signal as the transmission medium. The basic encoding of the data in the television signal is carried out in synchronism with the horizontal repetition rate of the video scan lines within the signal.
Due to the inherently wide bandwidth of television signals, they have proven to be quite capable as a medium for transmitting high speed data. However, in the event a relatively low speed input device is used, and the data or information rate of that device is to be left unaltered for transmission, the known systems are then under-utilized since, during a fixed time interval in which a large amount of digital information can be encoded onto the television signal, only a relatively small amount of data has been provided by the input device and only that data is encoded by the system onto the television signal. Further, in the event two or more input devices are used in the known systems, data from only one of the devices is encoded into a given video scan line at one time. For example, in the DIDON system, a packet address is encoded at the beginning of each scan line to identify a particular input device and, thus, to allow the encoded data to be routed to the proper receiving terminal. This allows 256 possible data channels to be handled by the DIDON system for vertical interval transmission, and up to 4,096 possible data channels in the event the DIDON system is used in a full-field mode. Each of the data channels, therefore, occupies at least an entire scan line since the packet address of that line operates to distribute the encoded data only within that channel. Use of a relatively low speed data input device thus would cause a great number of scan lines to be transmitted each beginning with the packet address associated with the channel of the device, but each having substantially less than the total amount of digital data which can be encoded on the line. Time which could otherwise have been allocated to the transmission of higher speed data is therefore lost as long as the lower speed device continues to transmit its data.